The present invention is based on a method of manufacturing piezoelectric actuators, in particular for fuel injection valves, according to the definition of the species of the main claim.
Piezoelectric actuators, in particular for actuating fuel injection valves, are known in a plurality of designs, for example from German Patent Application 195 00 706 A1 or German Patent 43 06 073 C1. Piezoelectric actuators have a plurality of piezoelectric layers stacked one above the other, each one being coated with an electrode on the surface. Piezo actuators normally have hundreds of piezoelectric layers thus layered one above the other. A relatively large actuating lift is achieved in this manner. As described in detail, for example, in German Patent Application 37 13 697 A1, the electrodes of the individual piezoelectric layers must be alternatingly connected to a voltage source in order to generate an electric field oriented in the same direction in the individual layers. Every second electrode is connected to a first pole of a voltage source, while the electrodes in between are connected to a second pole of a voltage source.
German Patent Application 195 34 445 A1 describes piezoelectric actuators having a central through passage.
In manufacturing piezoelectric actuators, the common procedure is to initially manufacture thin sheets made of a piezoelectric ceramic material and to coat the surface of these sheets with electrodes via vapor deposition or sputtering. Then the sheets are stacked one above the other and pressed together. Subsequently the stacked sheets are sintered so that the individual layers are baked together. Finally the stacks are cut, for example by sawing, to form individual actuators.
Since the actuators used for actuating fuel injection valves must have a central passage for the fuel in order to achieve a compact design, previously this central passage was usually made using a material-removing operation after stacking and sintering, usually by drilling. This material-removing operation is, however, costly, since the sintered ceramic layers have a relatively high hardness. Previously, in addition, the central passage produced by drilling had to be subjected to a finishing operation by machining in order to avoid short-circuits between the electrodes. It is furthermore particularly disadvantageous that only round passages can be produced by drilling. In practice, however, it may be necessary to produce passages having other shapes. By sawing apart the stacked and sintered sheets to produce the individual actuators, actuators with a rectangular cross-section are obtained. When these actuators are installed in fuel injection valves having a hollow cylindrically shaped housing contour, actuators having a cylindrically shaped external contour have the advantage of requiring little space. In order to produce cylindrically shaped actuators having a round cross-section from the rectangular actuators produced by sawing, the actuators must be post-processed by cutting or turning, which is relatively expensive and increases manufacturing costs.
The method of manufacturing piezoelectric actuators according to the present invention having the characterizing features of Claim 1 has the advantage over the related art that the central passage of the individual actuators can be produced in a simple manner so that suitable cutouts are produced in the sheets prior to stacking and sintering, these cutouts then forming the central passage of the actuators after stacking. For this purpose, the sheets are arranged exactly flush to each other one above the other, which can be easily achieved using centering holes provided through the sheets, for example. The exact alignment of the sheets one above the other helps create a central passage having a smooth internal wall, which requires no subsequent processing. Therefore, the method according to the present invention can be used to particular advantage in mass production. No material-removing operation is required.
Another important advantage is that not only circular passages, but also passages having other contours, can be produced.
The measures presented in the subclaims allow advantageous refinements of and improvements in the method described in Claim 1.
Using the method according to the present invention, not only the internal contours of the central passage, but also the outer contours of the actuators can be pre-shaped. For this purpose, each sheet is shaped prior to stacking so that these have a plurality of actuator areas which form the actuators after stacking, these areas being connected to a frame area which holds the actuator areas together. Outside the connecting webs, the actuator areas already have their final contour, the areas outside the actuator areas, the connecting webs, and the frame area being separated from the sheets by stamping or laser cutting, for example. It is particularly advantageous that round actuator outer contours, for example, can also be manufactured without the need for subsequent processing of the stacked and sintered actuators using a material-removing operation, for example, drilling or turning. Actuators having a round outer contour are particularly well-suited for use in fuel injection valves, since they can be installed in the hollow cylindrical valve housing in a compact manner.
The frame area of the sheets is advantageously made of an external frame and a plurality of crossbars arranged in a row, to which the actuator areas are attached via the connecting webs. After stacking and sintering of the sheets, the individual actuators are separated from the stacks by separating the connecting webs via laser cutting or breaking, for example. The web segments to be separated are relatively short.
The electrodes are advantageously applied to the sheets so that they are at a predefined distance from the cutouts and from the external contours of the actuators. This ensures that the electrodes do not extend to the external surface of the actuators or to the central passage, thus being isolated from the area surrounding the actuators and from the fuel flowing through the central passage.